Oxygen sensors made of alkaline-earth-doped lanthanum ferrites and method of use thereof

ABSTRACT

Oxygen sensor for measuring resistance as a function of oxygen partial pressure, made of alkaline-earth-doped perovskitic lanthanum ferrites with the general formulaLa1-xMexFeO3- delta where Me is one of alkaline earth metals, Mg, Ca, Sr, and Ba, x is a degree of doping of the lanthanum ferrites. The oxygen sensor oxygen deficit of anion is 6=0 to 0.25, and the degree of doping of the lanthanum ferrites is x=0.1 to 0.3 and is selected to provide a temperature independent resistance property to the oxygen sensor in a lean range.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of Ser. No. 08/737,623,filed Mar. 3, 1997, now U.S. Pat. No. 5,843,858, which is a nationalstage application of PCT/EP95/01755, filed May 9, 1995.

FIELD OF THE INVENTION

The present invention relates to oxygen sensors according to thepreamble of the main claim.

BACKGROUND OF THE INVENTION

Oxygen sensors that contain strontium ferrate, barium ferrate, orstrontium-barium ferrate are known (U.S. Pat. No. 4,454,494). In thesematerials, the central iron atoms of the ferrate lattice are replaced byone to seventy atom-percent of an element from the group composed oftitanium, cerium, tantalum, or niobium. The chemical stability limit ofthese materials is at a temperature of 850° C. with an O₂ partialpressure of pO₂ =10⁻¹⁸ bar, i.e. these materials decompose duringprolonged operation in a reducing atmosphere, in a rich exhaust mixturefor example.

The transition from p- to n- semiconducting sensor material in thematerials of U.S. Pat. No. 4,454,494 takes place at a relatively high O₂partial pressure (>10⁻¹⁰ bar). This results in ambiguity of the sensorsignal or insufficient signal at a gas change between rich and leanexhaust. In addition, the materials exhibit a clearly differenttemperature dependence for different O₂ partial pressure ranges.

An oxygen sensor is known (Chem. Abstr. 112 (1990), reference number126, 210t) whose sensor material consists of an alkaline-earth-dopedlanthanum ferrite. However, this sensor material is not used to measurethe change in resistance based on the recorded oxygen partial pressure,but the thermo electric force. In these sensor materials, a significanttemperature dependence prevails. To compensate for this temperaturedependence, additional measures must be adopted, temperature-regulatingmeasures or precise settings of currents and/or voltages for example.This document therefore contains technical prejudice against oxygensensors that use the resistive properties of the sensor material. Thethermo electromotive force is measured instead.

BRIEF SUMMARY OF THE INVENTION

The goal of the present invention is to provide improved sensormaterials for oxygen sensors that are suitable for applications inlambda sensors for exhaust from combustion processes. In particular,these materials should exhibit a temperature-independent sensorresistance in the range of lean exhaust mixtures (λ>1), while in therich area they are temperature-activated in such fashion that theinfluence of the temperature on the sensor resistance can be compensatedto the greatest degree possible by the temperature dependence of thethermodynamic equilibrium reactions of the gas components typicallycontained in the exhaust. As a result, the influence of temperature onthe fluctuation and resistance between rich and lean ranges can belargely eliminated.

This goal is achieved by oxygen sensors made of alkaline-earth-dopedperovskitic lanthanum ferrites according to the characterizing clause ofthe main claim.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 show measurement results with ferrite ceramics of theformula La₀.75 Sr₀.25 FeO₃ and La₀.5 Sr₀.5 FeO₃.

FIG. 3 shows that in the range of high partial pressures greater than10⁻⁴ bar, in a comparison between ceramic bulk material according toFIG. 2 and a thick film according to Example 3, there is a clearreduction of the temperature dependence.

FIG. 4 shows the dependence of specific resistance log (P Ωm⁻¹) on O₂partial pressure at temperatures of 800°, 900° and 1000° C.

FIGS. 5-8 show additional graphs of specific electrical conductivity(sigma) as a function of temperature and oxygen partial pressure forvarious doped lanthanum products.

FIG. 9 shows, an oxygen sensor in accordance with the present invention,including non-conducting metal oxide substrate 1, thick film ofalkaline-earth-doped perovskitic lanthanum ferrites 3, which includescontacts for measuring resistance.

DETAILED DESCRIPTION OF THE INVENTION

The alkaline-earth-doped lanthanum ferrites used in the oxygen sensorsaccording to the invention have the general formula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of the alkaline-earth metals Mg, Ca, Sr, and Br,especially Ca, Sr, and Ba, and the degree of doping x=0.1 to 0.3. Theoxygen deficit of the anion is δ=0 to 0.25.

It has been found that the material properties in suchalkaline-earth-doped lanthanum ferrites can be significantly improved bya careful choice of the mixed-ceramic composition, i.e. by varying theratio of lanthanum to alkaline earth. With a degree of doping in therange from 0.1 to 0.3, the ferrites exhibit a sensor characteristic thatis independent of temperature and pO₂, provided the oxygen partialpressure remains in the range from 10⁻³ to 10⁻² bar (lambda greater than1). In the rich range (lambda less than 1) these materials exhibit anincreased dependence on oxygen partial pressure and temperature. Thisinfluence of temperature on resistance however is largely compensated bythe temperature dependence of the oxygen partial pressure that isproduced by the thermodynamic equilibrium reactions of the exhaustcomponents. This results in an approximately constanttemperature-dependent sensor resistance in the range of rich exhaustmixtures.

This result is surprising, since U.S. Pat. No. 4,454,494 states thatincreased lanthanum doping of calcium ferrates results in an electricalconductivity that is too high and that these materials exhibit apronounced hysteresis of conductivity as a function of temperature.

The alkaline-earth-doped La ferrites used are produced for example fromLa₂ O₃, Fe₂ O₃, and alkaline earth carbonate, with the startingmaterials being mixed, calcined, and sintered. The solid body reactionthat proceeds at calcination temperature T_(K) is described by thefollowing chemical equation: ##EQU1## During this calcination reactionof the starting powder according to the above reaction equation, inaddition to emission of CO₂, there is also an emission of smallquantities of oxygen that results in an oxygen deficit δ in both thecalcinate and in the mixed ceramic obtained from it. The δ in thestarting powder is determined gravimetrically and varies for example asa function of the Sr content between 0.00≦δ≦0.25. The table belowprovides an overview of the oxygen deficits determined at roomtemperature as a function of the Sr content according to the formula

    La.sub.1-x Sr.sub.x FeO.sub.3-δ

    ______________________________________                                        Sr Content/x       δ = oxygen defictic                                  ______________________________________                                        0.00               0.000                                                        0.01 0.000                                                                    0.03 0.01                                                                     0.05 0.02                                                                     0.10 0.03                                                                     0.25 0.05                                                                     0.30 0.08                                                                     0.40 0.10                                                                     0.50 0.12                                                                   ______________________________________                                    

Measurements of electrical conductivity on bulk material indicate that δfor Sr contents below x=0.25 at temperatures up to 1000° C. and pO₂ =1bar changes only insignificantly. For mixed ceramics with an Sr contentof x=0.3 or more, additional oxygen emission takes place aboveapproximately T=600° C. (pO₂ =1 bar), increasing with Sr content. ForLa₀.50 Sr₀.40 FeO₃ for example, for T=1000° C. and pO₂ =1 bar, by meansof thermogravimetry, a "basic" deficit evident from the above table atroom temperature, an additional oxygen deficit of approximately 0.05 wasfound. This means that for La₀.60 Sr₀.40 FeO₃, at 1000° C. and pO₂ =1bar, one obtains a "total" δ of approximately δ=0.15.

The δ range of the ferrites obtained extends from 0.00 to x/2 (x=0-25mol%) and depends on temperature, alkaline earth content, and pO₂.

The La₂ O₃ used is preferably initially subjected to an upstreamroasting process in which it is annealed at temperatures of about 700 to1000° C. The La₂ O₃ can then be weighed stoichiometrically in the hotstate (200° C. to 300° C.) together with the other constituents. The rawpowder thus obtained is then mixed by grinding. The mixing time can lastseveral hours, three hours for example for strontium mixed ceramics. Thematerials are dried and presintered to the starting powder.

The starting powder thus obtained is ground and then dried. It can besubjected to a multistage compression process. The subsequent sinteringis performed at temperatures from 1250 to 1450° C.

The La ferrite powder thus produced can be processed with basic materialand/or diluent to form a paste and this paste can be applied using athick film technique, for example by screen printing, dipping, spraying,or injection, to a preferably nonconducting metal oxide substrate, Al₂O₃. They are dried and stoved. Sensors can be made in a very simplefashion using these techniques. The sensor materials are applied usingthick film technology to carrier and are then stoved and/or tempered.

Surprisingly, it has been found that when the sensor materials areapplied using thick film techniques, the temperature dependence of theresistance by comparison to corresponding bulk material with the samecomposition is much less. This follows from a comparison of bulkmaterial with corresponding material applied using thick film techniquesaccording to the examples below, with the correspondingtemperature-dependent curves being shown in FIG. 3.

The invention will now be described in greater detail with reference tothe examples.

EXAMPLE 1

Production of Sr-doped La-ferrite

To produce strontium-doped La ferrites, a raw powder composed of La₂ O₃,Fe₂ O₃, and SrCO₃ was used. The most important powder properties aresummarized in the table below:

    ______________________________________                                        Powder         Purity       d.sub.50 /μm                                   ______________________________________                                        La.sub.2 O.sub.3                                                                             >99.98%      <7 μm                                            Fe.sub.2 O.sub.3 >99.86% <15 μm                                            SrCO.sub.3 >99.88% 2-10 μm                                               ______________________________________                                    

The average particle size diameter is in the range of d₅₀ =2 μm-15 μmfor all powders.

Each stoichiometric weighing of the various constituents is preceded byan annealing process for the La₂ O₃ prior to preparation. Approximately30 grams of La₂ O₃ is heated at 5° C. per minute to T=830° C. andannealed at this temperature for at least five hours. Cooling then takesplace at 1° C./min in general.

La₂ O₃ is weighed stoichiometrically in the hot state (200° C.-300° C.)together with the remaining constituents. Approximately 50 grams of rawpowder is mixed in a planetary ball mill with 50 balls (diameter=10 mm)in 150 ml C₆ H₁₂ for three hours. After the mixed raw substances havedried (minimum twelve hours), they are presintered to the startingpowder at T=1230° C. in air. The calcination time is ten hours. Theheating rate is 10° C. per minute and the cooling rate follows thefurnace time constant.

The starting powder is likewise ground for two hours in the planetaryball mill together with 150 ml of C₆ H₁₂ and ten balls (diameter=20 mm)(average grain size d₅₀ =4-5 μm) and then dried again for at leasttwelve hours. All of the powders exhibit a single-phase composition asdetermined by x-ray diffractometry.

EXAMPLE 2

Sintering Ceramic Bulk Materials

Beginning with approximately 2.5 g of starting powder, initially in atwo-stage compression process (uniaxial precompression at 4 MPa, coldisostatic redensification with 220 MPa), green bodies with a greendensity of approximately 60% to 64% of the theoretical density arepressed. The sintering itself takes place at T=1400° C. (t_(s) =3 h),primarily in air at a heating rate of 10° C. per minute and a coolingrate of 5° C. per minute.

The sintering densities that can be reached with a preparation are in arange between 97% and 100% of the theoretically determined density. Themaximum foreign phase component found is less than 1%.

Determination of O₂ Partial Pressure and Temperature Dependence

To perform the measurements of the specific electrical conductivity,parallelipipedal samples of various geometries were produced from theceramic bulk material.

The contacts on the samples were obtained using the four-point methodwith platinum paste stoved for 1200° C. for twenty minutes.

Tests performed at room temperature using impedance spectroscopy show apurely ohmic electrical behavior for all ceramics.

The various ceramics were studied using a special sample carrier in atubular furnace sealed gas-tight. The temperatures vary between 800° C.and 1000° C. and the pO₂ pressure can be set between 10⁻²⁰ bar and 10⁰bar.

For measuring the specific electrical resistance, initially thetemperature equilibrium must be established in the measuring chamber.After setting the respective pO₂ pressure, the electrical resistance ofthe sample is determined within a fixed period of time (one to fourhours).

The measurement results with ferrite ceramics with the formulas La₀.75Sr₀.25 FeO₃ and La₀.5 Sr₀.5 FeO₃ are shown in FIGS. 1 and 2. While at Srdopings greater than 30% a pronounced temperature dependence is stillobserved in the range of high partial pressure (>10⁻³ bar, leanmixture), this temperature dependence is negligible for dopings below30%.

FIG. 4 shows the dependence of specific resistance log (P/Ωm⁻¹) on O₂partial pressure at temperatures of 800°, 900°, and 1000° C. The signalsin the rich and lean areas differ sharply, but are relatively constantin one of the respective ranges.

For comparison, additional graphs of specific electrical conductivity(sigma) as a function of temperature and oxygen partial pressure forvarious doped lanthanum products are shown in FIGS. 5 to 8. The nondopedlanthanum ferrite, especially in the left branches of the curve,exhibits a pronounced temperature dependence, while with a doping degreeof only 0.1 (with a variation at low temperature) the varioustemperature curves are nearly coincident. This is also the case with adegree of doping of 0.3 while with a higher degree of doping (0.5) thetemperature dependence becomes stronger once again as can be seen fromthe fact that the curves no longer coincide.

EXAMPLE 3

Production of Ceramic Thick Films

The La ferrite powders produced according to Example 1 are processedwith basic paste and/or diluent to form a paste and this paste isapplied by screen printing to a nonconducting metal oxide substrate madeof Al₂ O₃, dried for fifteen minutes at 120° C., and stoved to theunderlying sintered profile.

Increase at 20 K/min to 350° C.

Hold for 10 minutes at 350° C.

Increase at 20 K/min to 1200° C.

Hold for 25 minutes

Cool at 20 K/min to ambient temperature

As shown in FIG. 3, in the range of high partial pressures greater than10⁻⁴ bar, in a comparison between ceramic bulk material according toFIG. 2 and a thick film according to example 3, there is once again aclear reduction of the temperature dependence, which is slight to beginwith. This result is extremely surprising and could not be foreseen bythe individual skilled in the art, since normally at best the sametemperature effects are obtained for thick films and ceramics. In mostcases, when thick film technology is employed, there is a worse ratiorelative to temperature dependence and the characteristic curve bycomparison with ceramic bulk material.

We claim:
 1. A method of use of an oxygen sensor for measuringresistance as a function of oxygen partial pressure, wherein said oxygensensor is made of alkaline-earth-doped perovskitic lanthanum ferriteswith the general formula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of Mg, Ca, Sr, and Ba, x is a degree of doping of thelanthanum ferrites, an oxygen deficit of anion δ=0 to 0.25, and thedegree of doping of the lanthanum ferrites is x=0.1 to 0.3, and saidalkaline-earth-doped perovskitic lanthanum ferrites are selected toprovide a temperature independent resistance property to the oxygensensor in a lean range, of gas components in exhaust, and said sensorincludes contacts for measuring resistance, wherein said sensor is usedas a lambda sensor to determine an air/fuel ratio in exhaust from acombustion process, comprising the step of providing said oxygen sensorin a combustion process, measuring resistance of said oxygen sensor anddetermining the air/fuel ratio in exhaust from said combustion processbased on the measured resistance.
 2. The method of use of an oxygensensor for measuring resistance as a function of oxygen partialpressure, according to claim 1, wherein said alkaline-earth-dopedperovskite lanthanum ferrites are applied as a thick film to anon-conducting metal oxide substrate.
 3. An oxygen sensor for measuringresistance as a function of oxygen partial pressure, wherein said oxygensensor is made of alkaline-earth-doped perovskitic lanthanum ferriteswith the general formula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of Mg, Ca, Sr, and Ba, x is a degree of doping of thelanthanum ferrites, an oxygen deficit of anion δ=0 to 0.25, and a degreeof doping of the lanthanum ferrites is x=0.1 to 0.3, said sensorcomprising a non-conducting metal oxide substrates, wherein saidalkaline-earth-doped perovskitic lanthanum ferrites are applied as athick film to said non-conducting metal oxide substrate, and contactsare positioned on said thick film of said alkaline-earth-dopedperovskitic lanthanum ferrites for measuring resistance as a function ofoxygen partial pressure, said oxygen sensor produced by a processcomprising the steps of: applying said alkaline-earth-doped perovskiticlanthanum ferrites to said substrate using thick film technology andstoving or tempering; or stoving and tempering said alkaline-earth-dopedperovskitic lanthanum ferrites applied to said substrate.
 4. A method ofusing an oxygen sensor for measuring resistance as a function of oxygenpartial pressure, said oxygen sensor comprising a lamba sensor fordetermining an air/fuel ratio in exhaust from combustion processes,comprising the steps of:providing, in a combustion process, an oxygensensor made of alkaline-earth-doped perovskitic lanthanum ferrites withthe general formula:

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of the alkaline earth metals, Mg, Ca, Sr, and Ba, X is adegree of doping of the lanthanum ferrites, an oxygen deficit of anionδ=0 to 0.25, and the degree of doping of the lanthanum ferrites is x=0.1to 0.3 and said alkaline-earth-doped perovskitic lanthanum ferrites areselected to provide a temperature independent resistance property to theoxygen sensor in a lean range, measuring resistance of said oxygensensor and determining the air/fuel ratio in exhaust from saidcombustion process based on the measured resistance.
 5. An oxygen sensorcomprising alkaline-earth-doped perovskitic lanthanum ferrites with thegeneral formula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is selected from the group consisting of Mg, Ca, Sr, and Ba,and x is a degree of doping of the lanthanum ferrites, and an oxygendeficit of anion δ=0 to 0.25, characterized in that the degree of dopingof the lanthanum ferrites is x=0.1 to 0.3, said sensor comprising anon-conducting metal oxide substrate, wherein said alkaline-earth-dopedperovskitic lanthanum ferrites are applied as a thick film to saidnon-conducting metal oxide substrate, and contacts are positioned onsaid thick film of said alkaline-earth-doped perovskitic lanthanumferrites for measuring resistance as a function of oxygen partialpressure.
 6. An oxygen sensor for measuring resistance as a function ofoxygen partial pressure, wherein said oxygen sensor is made ofalkaline-earth-doped perovskitic lanthanum ferrites with the generalformula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of Mg, Ca, Sr, and Ba, X is a degree of doping of thealkaline-earth-doped perovskitic lanthanum ferrites, an oxygen deficitof anion δ=0 to 0.25, and the degree of doping of thealkaline-earth-doped perovskitic lanthanum ferrites is x=0.1 to 0.3 andsaid alkaline-earth-doped perovskitic lanthanum ferrites are selected toprovide a temperature independent resistance property to the oxygensensor in a lean range of gas components present in exhaust, said sensorcomprising a non-conducting metal oxide substrate, wherein saidalkaline-earth-doped perovskitic lanthanum ferrites are applied as athick film to said non-conducting metal oxide substrate, and contactsare positioned on said thick film of said alkaline-earth-dopedperovskitic lanthanum ferrites for measuring resistance as a function ofoxygen partial pressure.
 7. The oxygen sensor according to claim 6,wherein the oxygen sensor exhibit a temperature-independent sensorresistance in a lean range (lambda>1) and is temperature-activated in arich range (lambda<1), wherein the influence of temperature on saidsensor resistance is compensated by temperature dependence ofthermodynamic equilibrium reactions of gas components contained in theexhaust in the rich range.
 8. The oxygen sensor according to claim 6,wherein the degree of doping x=0.1 to 0.25.
 9. The oxygen sensoraccording to claim 6, wherein the degree of doping is x=0.1 to 0.2. 10.The oxygen sensor according to claim 6, wherein said oxygen sensor hasan oxygen partial pressure greater than 10⁻⁴ bar.
 11. An oxygen sensorfor measuring resistance as a function of oxygen partial pressure, saidsensor made of a alkaline-earth-doped perovskite lanthanum ferrites withthe general formula

    La.sub.1-x Me.sub.x FeO.sub.3-δ

where Me is one of the alkaline earth metals, Mg, Ca, Sr, and Ba, x is adegree of doping, the oxygen deficit if anion δ=0 to 0.25, and thedegree of doping of the lanthanum ferrites is x=0.1 to 0.3, wherein saidalkaline-earth-doped perovskitic lanthanum ferrites are selected toprovide a temperature independent resistance property to the oxygensensor in a lean range, said sensor comprising a non-conducting metaloxide substrate, wherein said alkaline-earth-doped perovskitic lanthanumferrites are applied as a thick film to said non-conducting metal oxidesubstrate, and contacts are positioned in said thick film of saidalkaline-earth-doped perovskitic lanthanum ferrites for measuringresistance as a function of oxygen partial pressure, and wherein saidoxygen sensor is produced by a process comprising the steps of applyingsaid alkaline-earth-doped perovskitic lanthanum ferrites, using thickfilm technology to said non-conducting metal oxide substrate, and dryingsaid thick film on said non-conducting metal oxide substrate for 15minutes at a first temperature of 120° C., increasing said firsttemperature at 20K/min to a second temperature of 350° C. and holdingsaid second temperature for 10 minutes at 350° C.; increasing saidsecond temperature at 20K/min to a third temperature of 1200° C. andholding said third temperature for 25 minutes at 1200° C., and coolingsaid thick film on said non-conducting metal oxide substrate at 20K/minto ambient temperature.